Nonaqueous electrolyte secondary cell and a tungsten or molybdenum substituted lithium positive electrode active material

ABSTRACT

A positive active material for nonaqueous electrolyte secondary batteries which has a higher capacity and improved thermal stability in a charged state and is less expensive compared to the current active material of LiCoO 2  is provided by a lithium compound oxide having the formula: 
     
       
         Li a Ni b Co c Mn d M e O 2   (1) 
       
     
     where M stands for one or two of W and Mo, 
     
       
         0.90≦ a ≦1.15, 0&lt; b &lt;0.99, 0&lt; c ≦0.5, 0&lt; d ≦0.5, 0&lt; c+d ≦0.9, 0.01≦ e ≦0.1, and  b+c+d+e =1, 
       
     
     the lithium compound oxide giving an X-ray diffraction pattern including a diffraction peak or peaks assigned to a compound oxide of Li and W and/or a compound oxide of Li and Mo, in addition to main diffraction peaks assigned to a hexagonal crystal structure.

TECHNICAL FIELD

The present invention relates to a positive active material fornonaqueous electrolyte secondary batteries useful as a power source inportable electronic or communications equipment, electric cars, and thelike, and to a nonaqueous electrolyte secondary battery using thepositive active material.

BACKGROUND ART

Lithium ion secondary batteries, which are one class of nonaqueouselectrode secondary batteries, have advantages including a high voltage,a high energy density, and a low self discharge, and they have becomeindispensable as a power source for portable electronic orcommunications equipment such as mobile phones, laptop personalcomputers, camcorders, and the like.

Lithium ion secondary batteries which are currently in practical use are4V-grade batteries in which a carbon material such as graphite is usedfor the negative electrode, LiCoO₂ (lithium cobaltate) is used as anactive material for the positive electrode, and a nonaqueous solution ofa lithium salt in an organic solvent is used as the electrolyticsolution. When these batteries are charged, the following reactionoccurs:

Charge reaction: LiCoO₂+nC₆→Li_((1-n))CoO₂+nLiC₆.

A high voltage of at least 4.8 V is required for full (100%) charge ofthe batteries. However, such a high voltage may cause decomposition ofthe electrolytic solution and adversely affect the reversibility ofcharge and discharge reactions, resulting in a loss of the cycle life ofthe secondary batteries. Therefore, in practice, the maximum voltage islimited to 4.1-4.2 V. Thus, the positive active material is utilized ina stable region where the value of “n” in the above reaction is around0.5, so the positive active material as charged can be expressedapproximately as Li_(0.5)CoO₂.

As the performance of portable electronic or communications equipment isincreased, it is increasingly required for secondary batteries to have ahigh energy density with a small size and a light weight. On the otherhand, in the field of large-sized secondary batteries for use inelectric cars which are under development with a view of maintaining theglobal environment, there is a demand for secondary batteries which notonly have a high energy density but also are safe.

In addition, the costs of secondary batteries are important,particularly for large-sized secondary batteries. The use of anexpensive cobalt compound whose resources are limited as a positiveactive material necessarily adds to the cost of the above-describedpractical lithium ion secondary batteries. The high cost of lithium ionsecondary batteries is a major cause when they are precluded from beingmounted in electric cars.

It is well known that LiNiO₂ (lithium nickelate) can also be used as apositive active material for lithium ion secondary batteries. LikeLiCoO₂, LiNiO₂ has a layered, hexagonal crystal structure and allowslithium (Li) ions to be intercalated and deintercalated between layersof the crystal structure. The charge reaction occurring when LiNiO₂ isused as a positive active material is basically the same as theabove-described charge reaction for LiCoO₂. However, LiNiO₂ can becharged in a stable manner until the value of “n” in the above chargereaction formula reaches around 0.7. Thus, in this case, the positiveactive material as charged can be expressed approximately asLi_(0.3)NiO₂, thereby constituting a positive electrode of a highercapacity.

Compared to LiCoO₂, LiNiO₂ has the advantages of being less expensiveand being capable of making a secondary battery of higher capacity.However, LiNiO₂ has the problem that its crystal structure tends to bebroken during charging and discharging, thereby adversely affecting thecycle properties of the secondary battery. In addition, in a secondarybattery having a positive active material of LiNiO₂, an exothermicdecomposition reaction may occur when the charged positive activematerial is exposed to a high temperature in the presence of theelectrolytic solution, whereby the active material is converted into acompound approximately expressed as Li₂Ni₈O₁₀ and oxygen is liberated.The liberated, active oxygen may react with the electrolytic solution oranother component, or serve as combustion-promoting oxygen. As a result,there may be a risk of igniting the battery itself in some cases. Thus,a secondary battery using LiNiO₂ as a positive active material has poorthermal stability, and this positive active material could not be usedin practical batteries.

As an attempt to improve the cycle properties of a secondary batteryusing LiNiO₂ as a positive active material, stabilizing the crystalstructure of this compound by replacing part of Ni by another elementsuch as Co was investigated, as described in Solid State Ionics, 90, 83(1996). This approach makes it possible to considerably improve thecycle properties.

On the other hand, with respect to the thermal stability of a secondarybattery using LiNiO₂ as a positive active material, it was reported inthe 40th Symposium on Batteries in Japan (1999), Presentation Number1C12 that when part of Ni is replaced by Co+Mn, the thermal stabilitycan be improved as the amount of replacing Co increases or the Nicontent decreases with a certain amount of replacing Mn. With thisapproach, however, it is difficult to improve the thermal stability tothe same level as that of LiCoO₂, although an initial capacitysurpassing that of LiCoO₂ can be obtained.

A LiCoO₂-based positive active material containing at least one elementselected from Cu, Zn, Nb, Mo, and W is described in JP-A 06-283174.Although it is explained therein that the positive active material has ahigh capacity and good cycle properties, the cycle properties aremeasured with only ten cycles and do not yet reach a level sufficientfor practical use.

As discussed above, with LiNiO₂-based materials which are positiveactive materials less expensive than LiCoO₂, although it is possible toattain a high capacity surpassing that attainable with LiCoO₂, thethermal stability of the positive active materials in their chargedstate is poor, and it is difficult to improve the thermal stability tothe same level as that of LiCoO₂. Thus, none of these materials havebeen improved in both initial capacity and thermal stability.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to develop a LiNiO₂-basedpositive active material for use in nonaqueous electrolyte secondarybatteries which has an initial capacity higher than that of LiCoO₂ andwhich is improved in thermal stability in a charged state at least tothe same level as LiCoO₂, thereby making it possible to providenonaqueous electrolyte secondary batteries which are less expensive andhave better performance than the current practical lithium ion secondarybatteries.

The present inventors found that the thermal stability of a LiNiO₂-basedpositive active material can be improved at least to the same level asthat of LiCoO₂ by replacing part of the Ni in LiNiO₂ by Co and Mn andfurther by one or both of W and Mo. Although the invention is notintended to be bound by a specific theory, it is presumed that theimprovement in thermal stability results from suppression of oxygenliberation which is caused by decomposition of the positive activematerial during charging and also from shifting the decompositiontemperature to a higher temperature.

The present invention is a positive active material for use innonaqueous electrolyte secondary batteries, characterized in that it iscomprised of a lithium compound oxide (compound oxide of lithium) of theformula:

Li_(a)Ni_(b)Co_(c)Mn_(d)M_(e)O₂  (1)

where M stands for one or two of W and Mo,

0.90≦a≦1.15, 0<b<0.99, 0<c≦0.5, 0<d≦0.5, 0<c+d≦0.9, 0.01≦e≦0.1, andb+c+d+e=1,

and in that the lithium compound oxide gives an X-ray diffractionpattern including a diffraction peak or peaks assigned to a compoundoxide of Li and W and/or a compound oxide of Li and Mo, in addition tomain diffraction peaks assigned to a hexagonal crystal structure.

The present invention also relates to a nonaqueous electrolyte secondarybattery comprising a negative electrode comprised of lithium metal or asubstance capable of absorbing and desorbing Li or Li ions and apositive electrode comprised of the above-described positive activematerial.

The positive active material for nonaqueous electrolyte secondarybatteries having a composition shown by the above formula (1) accordingto the present invention has significantly improved thermal stability ina charged state. As can be seen from the DSC (differential scanningcalorimeter) diagrams given in the examples described below, theimproved thermal stability is not only superior to that of LiNiO₂, butis also superior to that of conventional thermally-stabilizedLiNiO₂-based positive active materials in which part of Ni is replacedby Co and Mn, and is even superior to that of LiCoO₂ which is inherentlythermally stable. Thus, addition of an extremely small amount of Wand/or Mo provides a significant improvement in thermal stabilitywithout a significant decrease in initial capacity. The effect of Wand/or Mo on improvement in thermal stability is not known and has beenfirst found by the present inventors. As a result, the present inventionmakes it possible to manufacture nonaqueous electrolyte secondarybatteries having both good thermal stability and good initial capacity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an X-ray diffraction pattern of a lithium compound oxideobtained in Example 1 as a positive active material according to thepresent invention (in the upper half) and a partial enlarged view of theupper chart (in the lower half);

FIG. 2 shows an X-ray diffraction pattern of a lithium compound oxideobtained in Example 2 as a positive active material according to thepresent invention (in the upper half) and a partial enlarged view of theupper chart (in the lower half);

FIG. 3 shows an X-ray diffraction pattern of a lithium compound oxideobtained in Example 3 as a positive active material according to thepresent invention (in the upper half) and a partial enlarged view of theupper chart (in the lower half);

FIG. 4 shows an X-ray diffraction pattern of a lithium compound oxideobtained in Comparative Example 1 as a positive active material (in theupper half) and a partial enlarged view of the upper chart (in the lowerhalf);

FIG. 5 shows an X-ray diffraction pattern of a lithium nickelateobtained in Comparative Example 2 as a positive active material (in theupper half) and a partial enlarged view of the upper chart (in the lowerhalf);

FIG. 6 shows an X-ray diffraction pattern of a lithium cobaltateobtained in Comparative Example 3 as a positive active material (in theupper half) and a partial enlarged view of the upper chart (in the lowerhalf);

FIG. 7 shows DSC diagrams of compound oxides in a charged state whichwere obtained in Examples 1 to 3 and Comparative Examples 1 to 5 aspositive active materials; and

FIG. 8 is a schematic illustration showing the structure of acoin-shaped battery assembled in the examples for a battery test.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The positive active material for nonaqueous secondary batteriesaccording to the present invention has a composition as shown in theabove formula (1).

In compositional formula (1), the molar ratio of Li, “a”, is between0.90 and 1.15. If the value of “a” is less than 0.90 or greater than1.15, Ni or other transitional elements may enter the 3 b sites (Lisites) of the layered hexagonal crystal lattice of the lithium compoundoxide, thereby causing the discharge capacity of the material todecrease. The value of “a” is preferably between 0.95 and 1.10 and morepreferably between 0.99 and 1.10.

The sum of the molar ratios of the metals other than Li (i.e., b+c+d+e)is 1.

Nickel (Ni) serves to form the hexagonal crystallographic skeleton ofLiNiO₂, which provides a high discharge capacity. The molar ratio of Ni,“b”, is the remainder of the sum of the molar ratios of the metals otherthan Li [i.e., b=1−(c+d+e)]. The value of “b” is greater than 0 and lessthan 0.99.

Cobalt (Co) and manganese (Mn) are present in order to improve thethermal stability of LiNiO₂. The molar ratio of Co, “c”, and that of Mn,“d”, are both greater than 0 and not greater than 0.5. The value of thesum of (c+d) is greater than 0 and not greater than 0.9. For the purposeof improving thermal stability, it is advantageous to add both Co andMn. However, if the value of either “c” or “d” exceeds 0.5 or the valueof the sum of (c+d) exceeds 0.9, the proportion of the LiNiO₂ skeletonwhich exhibits a high capacity decreases so as to cause an extremedecrease in the discharge capacity. Preferably, the value of “c” is atmost 0.4, the value of “d” is at most 0.3, and the value of the sum of(c+d) is at most 0.7.

In order to ensure that the thermal stability is improved sufficiently,it is preferred that Co and Mn be added as follows: c≧0.05, d≧0.01. Mostpreferably, the value of“c” is between 0.10 and 0.35, the value of“d” isbetween 0.05 and 0.30, and the value of the sum of (c+d) is between 0.25and 0.65.

The addition of Co and Mn is not enough to improve the thermal stabilityof LiNiO₂ sufficiently. In accordance with the present invention, thethermal stability of LiNiO₂ is significantly improved by further addingone or both of tungsten (W) and molybdenum (Mo). In order to attain thiseffect, the value of the molar ratio of M (which stands for W and/orMo), “e”, is between 0.01 and 0.1. If the value of “e” is less than0.01, the thermal stability is not improved sufficiently. If it isgreater than 0.1, the discharge capacity is deteriorated. The value of“e” is preferably less than 0.05 and more preferably at least 0.02 andless than 0.05. Thus, W and/or Mo exhibits a significant thermalstability-improving effect when added in a very small amount.

A positive active material comprised of a lithium compound oxide of theabove formula (1) according to the present invention can be prepared byany appropriate method. A common method comprises mixing oxides of theindividual metal elements or their precursors (i.e., substances capableof forming the desired metal oxides by decomposition or oxidation) at apredetermined ratio as uniformly as possible and calcining the resultingmixture in an oxidizing atmosphere. This method is described below morespecifically, but it should be understood that the positive activematerial according to the present invention can be prepared by a methodother than the described one.

Ni, Co, and Mn can be coprecipitated as carbonate salts, for example, togive a compound carbonate salt in which these metal elements are mixeduniformly at their atomic level. Specifically, an aqueous solutioncontaining at least one water-soluble compound of each of these metals(e.g., an aqueous solution containing sulfates of these metals) at apredetermined atomic ratio is mixed with an alkaline solution ofammonium hydrogen carbonate at room temperature or under warming,thereby causing these metals (metal ions) to coprecipitate as carbonatesalts and form a compound carbonate of Ni, Co, and Mn. This reaction ispreferably carried out by adding to a reactor the aqueous solution ofthe metal compounds and that of ammonium hydrogen carbonate both insmall portions simultaneously or alternatingly in order to facilitate auniform crystal growth. The resulting compound carbonate may beconverted into the corresponding compound oxide by heating at atemperature of 300-900° C. to cause thermal decomposition fordecarboxylation. The metal compounds used as starting materials are notlimited to sulfates, and other appropriate compounds such as chlorides,nitrates, and acetates, which are soluble in water or in an acidsolution, may be used.

The compound carbonate of Ni, Co, and Mn obtained by the above-describedmethod or the compound oxide thereof obtained from the compoundcarbonate by heating is then uniformly mixed with a Li source and asource or sources of W and/or Mo (e.g., using an appropriate mixingmachine), and the mixture is calcined in an oxidizing atmosphere toobtain a lithium compound oxide serving as a positive active materialaccording to the present invention. The calcination is carried out undersuch conditions that the Li source reacts with the sources of the otherconstituent metals (which are all transition metals) to form a compoundoxide of Li with the transition metals.

As the Li source, a lithium compound is more suitable than lithiummetal, which is too active. Useful lithium compounds include lithiumhydroxide (anhydrous or monohydrate), lithium carbonate, lithiumnitrate, lithium oxide, and the like. The source of each of W and Mo maybe either the metal itself or a metal compound such as an oxide,carbide, or chloride. Alternatively, W and/or Mo may be introduced intothe above-described compound carbonate by coprecipitation as a carbonatefrom a soluble compound along with Ni, Mn, and Co.

The metal sources present in the mixture to be calcined have anincreased reactivity as their particle diameters decrease. In thisrespect, the compound carbonate or compound oxide of Ni, Co, and Mn (andoptionally W and/or Mo) preferably has an average particle diameter offrom 6 to 20 μm, while the Li source compound and the sources of Wand/or Mo (if any is used) preferably have an average particle diameterof from 1 to 20 μm. Therefore, each source may be pulverized and/orclassified (sieved) as required.

The temperature at which the mixture is calcined is determined such thatthe Li source reacts with the sources of the transition metals to form acompound oxide of Li with the transition metals. A preferablecalcination temperature is normally in the range of 600-1100° C., andmore preferably in the range of 800-1050° C. The calcination atmosphereis an oxidizing atmosphere and preferably an atmosphere having an oxygenconcentration higher than that of air, and even a pure oxygen atmospheremay be used. The calcination is performed until the reaction between Liand the transition metals is completed. The duration of calcination isnormally at least a few hours depending on the temperature andatmosphere for calcination.

The positive active material (lithium compound oxide) according to thepresent invention, which is a product obtained by calcination, has acrystal structure in which the basic crystallographic skeleton ishexagonal, which is characteristic of LiCoO₂ and LiNiO₂. Thus, the 3 asites of the hexagonal lattices are occupied by Ni and Co, part of whichis further replaced by Mn.

The state of W and/or Mo as additive elements by which the presentinvention is characterized has not been completely elucidated, but in anX-ray diffraction pattern of the active material, one or morediffraction peaks assigned to a compound oxide of Li with Mo or W can beconfirmed. Therefore, at least part of W and/or Mo crystallizes out as acompound salt with Li to form different phases from the skeletalhexagonal crystals.

Thus, the lithium compound oxide according to the present invention ischaracterized in that it produces an X-ray diffraction pattern whichincludes, in addition to main diffraction peaks assigned to thehexagonal crystal structure of the basic skeleton, one or morediffraction peaks assigned to a compound oxide of Li and W and/or acompound oxide of Li and Mo (the latter peaks being hereinafter referredto as secondary diffraction peaks in some places). With a positiveactive material which does not produce such a secondary diffractionpeak, an improved thermal stability cannot be obtained even if itcontains W or Mo.

Although the particular compounds for a compound oxide of Li and W andthat of Li and Mo are not limited, they are typically Li₂WO₄ and Li₄MoO₅both having a rhombohedral crystal form. Thus, typically, one or morediffraction peaks assigned to such a rhombohedral crystal appear in anX-ray diffraction pattern of a positive active material according to thepresent invention.

A positive active material according to the present invention can beused to produce positive electrodes in a conventional manner. A mixturefor making positive electrodes (composition for forming positiveelectrodes) which predominantly comprises the positive active materialin powdered form is usually prepared. The mixture usually contains abinder and a conducting additive in addition to the positive activematerial. A positive electrode can be produced by forming a paste from amixture containing these constituents with a small amount of a solventand applying the paste to an electrode substrate serving as a currentcollector to form a thin layer of the paste, which is then dried,optionally after compacting by rolling or similar technique. Instead ofthe paste being applied, the paste may be preformed into a sheet, whichis then press-bonded to an electrode substrate and dried to produce apositive electrode. Other methods may be employed.

The binder is not critical unless it is attacked by the nonaqueouselectrolytic solution of a battery, and it is generally a fluoroplastic.The conducting additive is not always necessary, but it is usually addedsince the electric conductivity of the positive active materialaccording to the present invention is not considerably high. As aconducting additive, a carbon powder such as acetylene black isgenerally used. The electrode substrate may be made of a metal such asaluminum or a stainless steel, and it may be either a solid sheet or ina porous form such as a perforated sheet or mesh. The substrate may be avery thin sheet such as a foil.

When a nonaqueous electrolyte secondary battery according to the presentinvention is assembled using a positive electrode produced from apositive active material according to the present invention, the otherelements of the battery such as a negative electrode, electrolyticsolution, and separator are not limited to particular forms. Anonaqueous electrolyte secondary battery is represented by a lithiumsecondary battery, and a positive electrode produced from a positiveactive material according to the present invention is suitable for usein a lithium secondary battery, although, in principle, it can be usedin other nonaqueous electrolyte secondary batteries.

In the case of a lithium secondary battery, a negative electrode fornonaqueous electrolyte secondary batteries is comprised of eitherlithium metal or a substance capable of reversibly absorbing anddesorbing Li or Li ions. Lithium metal can be used to form a negativeelectrode for an experimental battery, but it is inevitably accompaniedby a loss of cycle life due to precipitation of lithium metal indendrites during charging. Therefore, in a battery for practical use,the negative electrode is normally formed from a substance capable ofreversibly absorbing and desorbing Li or Li ions. Examples of such asubstance for negative electrodes include carbonaceous materialsincluding pyrolytic carbon, coke (pitch coke, needle coke, petroleumcoke, etc.), graphite, vitreous carbon, fired organic polymers (obtainedby firing a phenolic, furane, or similar resin at an appropriatetemperature so as to carbonize it), carbon fibers, and activated carbon;lithium alloys (e.g., Li—Al alloys); and polymers such as polyacenes andpolypyrrols.

An electrolytic solution for a lithium secondary battery is usually anonaqueous solution of a lithium salt as an electrolyte dissolved in anorganic solvent at a concentration of about 0.5-1.5 M.

Examples of suitable electrolytes include lithium perchlorate, lithiumtrifluoromethane-sulfonate, lithium tetrafluoroborate, lithiumhexafluorophosphate, lithium hexafluoroarsenate, and the like.

The organic solvent used to dissolve the electrolyte includes, forexample, propylene carbonate, ethylene carbonate, butylene carbonate,γ-butyrolactone, dimethyl carbonate, ethyl methyl carbonate, acetatecompounds, propionate compounds, diacetate compounds, dimethoxyethane,diethoxyethane, dimethoxypropane, diethoxypropane, tetrahydrofuran,dioxolanes, and the like, which may be used singly or as a mixed solventof two or more of these solvents.

In the future, it will be possible to use a polymeric electrolyte whichis now under development including a polymer complex type electrolytehaving Li ions coordinated to oxygen atoms of a polymer to form acomplex or a gel type electrolyte.

The shape and operating potential grade of the nonaqueous electrolytesecondary battery are not critical. The battery may be of any shape, soit may be a coin-shaped battery, cylindrical spiral-type battery, flatrectangular battery, inside-out cylindrical battery, polymer battery, orthe like. With respect to the operating potential grade, it is possibleto constitute a battery of up to max. 5V grade depending on thecombination with the material for the negative electrode.

Using a positive active material according to the present invention, itis made possible to manufacture nonaqueous electrolyte secondarybatteries having a higher capacity and a comparable or higher thermalstability with lower costs, compared to current lithium ion secondarybatteries using LiCoO₂ as a positive active material.

Furthermore, the nonaqueous electrolyte secondary batteries according tothe present invention can also be used suitably as large-sized batteriesfor automobiles or of the stationary type, since they can provide a highvoltage with a high energy density and have a good cycle life at hightemperatures.

EXAMPLES

The present invention will be illustrated by the following examples,which are presented merely for illustrative purpose and are not intendedto be restrictive in any way.

Example 1

Preparation of Compound Oxide

An aqueous solution A was prepared by dissolving nickel sulfate,manganese sulfate, and cobalt sulfate in purified water in such aproportion that the molar ratio of Ni:Mn:Co was 0.56:0.30:0.14.Separately, an aqueous solution B was prepared by adding a concentratedaqueous ammonia to an aqueous solution of ammonium hydrogen carbonate inan amount sufficient to make the solution alkaline. To an agitated tankcontaining an appropriate amount of water, the aqueous solutions A and Bwere alternatingly added in small portions at a given flow rate usingmetering pumps. After the addition was finished, the precipitates whichwere formed were collected by filtration, washed with water, and driedat 60° C. for one day to give a Ni—Mn—Co compound carbonate.

The compound carbonate was thermally decomposed by heating at around550° C. in air to give a Ni—Mn—Co compound oxide. The resulting compoundoxide had an average particle diameter of about 10 μm and was directlyused in the preparation of a positive active material withoutpulverization and classification.

Preparation of Positive Active Material

To 28.34 kg of the compound oxide, 9.16 kg of lithium hydroxide(anhydrous) and 2.5 kg of tungsten trioxide were added and uniformlymixed. The lithium hydroxide used was a fine powder obtained bypulverization in an oscillating ball mill followed by classification tocollect particles of 20 μm or smaller. The tungsten trioxide was also afine powder of 1 to 20 μm. The resulting powder mixture was placed in analuminum vessel having a purity of 99.8% and calcined for 10 hours at atemperature of 920-950° C. in a pure oxygen atmosphere to give 36.67 kgof a lithium compound oxide as a positive active material.

The composition of the resulting lithium compound oxide was analyzed byICP emission spectrometry and atomic absorption spectrometry anddetermined to contain 7.1% Li, 30.8% Ni, 16.0% Mn, 8.3% Co, and 4.6% Win mass % and have a molar ratio of Li/(Ni+Mn+Co+W) of 1.04. The lithiumcompound oxide had the composition shown in Table 1.

FIG. 1 shows an X-ray diffraction pattern of the lithium compound oxideusing Cu Kα rays. As can be confirmed from FIG. 1, the X-ray diffractionpattern includes, in addition to main diffraction peaks capable of beingindexed under space group R3-m (hexagonal system) (peaks assigned toLiNiO₂ or LiCoO₂), secondary diffraction peaks at 19.76°, 20.98°, and23.54° assigned to Li₂WO₄ of space group R-3 (rhombohedral system).

Testing Methods of Positive Active Material

(1) Battery Test

The lithium compound oxide (positive active material) prepared above,acetylene black (conducting additive), and a polytetrafluoroethyleneresin (binder) were placed in a mortar at a mass ratio of activematerial:conducting additive:binder of 67:22:11 and mixed for 15 minutestherein. The mixture was molded into a disc having a thickness of 0.2 mmand a diameter of 18 mm, which was press-bonded to a stainless meshhaving a diameter of 18 mm and dried at 200° C. to provide a positiveelectrode to be tested.

The positive electrode prepared above, a polypropylene separator (soldunder the tradename Cellguard), and a lithium metal foil measuring 0.2mm in thickness (negative electrode) were stacked in a test cell asshown in FIG. 8. As an electrolytic solution, a 1M solution of lithiumhexafluorophosphate (LiPF₆) in a mixed solvent of ethylene carbonate(EC) and dimethoxy carbonate (DMC) at a volume ratio of 1:2 was used andpoured into the test cell.

A repeated charge-discharge test was performed using a coin-shaped testbattery having the above described structure. The charge and dischargewere carried out under voltage regulations by charging to 4.3 V anddischarging to 3.0 V with a constant current of 0.707 mA (0.4 mA/cm²).The discharge capacity obtained after the first charging under theabove-described voltage regulations was recorded as an initial capacity.The charge and discharge were repeated for 50 cycles under theabove-described conditions, and the discharge capacity obtained in the50th cycle was also measured. The percent retention of the 50th cycledischarge capacity relative to the initial capacity was calculated toevaluate the cycle life of the test battery. The test results are alsogiven in Table 1.

(2) DSC Measurement (Thermal Stability in Charged State)

A coin-shaped battery was assembled in the same manner as described for(1) above. After being charged to 4.3 V, the battery was disassembled.The disc of the positive active material in a charged state wasrecovered from the positive electrode and thoroughly washed withdimethoxy carbonate (DMC). A 2 mg aliquot of the positive activematerial was placed into a stainless steel pressure pan for DSCmeasurement along with about 2 μL of the same electrolytic solution asabove [1M LiPF₆ in (EC+DMC, 1:2)] and DSC measurement was carried outwhile the temperature was increased from 25° C. to 500° C. at a rate of10° C./min. The results are shown in Table 1 and FIG. 7.

Example 2

To 29.04 kg of a Ni—Mn—Co compound oxide (Ni:Mn:Co=0.56:0.30:0.14)prepared in the same manner as described in Example 1, 9.39 kg oflithium hydroxide (anhydrous) and 1.57 kg of molybdenum trioxide wereadded and uniformly mixed. The lithium hydroxide was the same finepowder of 20 μm or smaller as used in Example 1, and the molybdenumtrioxide was also a fine powder of 1 to 20 μm. The resulting powdermixture was placed into an aluminum vessel having a purity of 99.8% andcalcined in the same manner as described in Example 1 to give 36.92 kgof a lithium compound oxide as a positive active material.

Analysis of the composition of the resulting lithium compound oxideshowed that it contained 7.2% Li, 30.9% Ni, 16.1% Mn, 8.3% Co, and 2.8%Mo in mass % and had a molar ratio of Li/(Ni+Mn+Co+Mo) of 1.05. Thelithium compound oxide had the composition shown in Table 1.

An X-ray diffraction pattern of the lithium compound oxide using Cu Kαrays is shown in FIG. 2. It can be seen that the X-ray diffractionpattern includes, in addition to main diffraction peaks capable of beingindexed under space group R3-m, a secondary diffraction peak at 21.08°assigned to Li₄MoO₅ of the rhombohedral system.

The results of measurements with a test battery prepared in the samemanner as described in Example 1 and the results of DSC measurements areshown in Table 1 and FIG. 7.

Example 3

In the same manner as described in Example 1 except that 9.29 kg oflithium hydroxide (anhydrous) and 1.71 kg of tungsten trioxide wereadded to 29.01 kg of a Ni—Mn—Co compound oxide(Ni:Mn:Co=0.56:0.30:0.14),36.56 kg of a lithium compound oxide wereobtained.

Analysis of the composition of the resulting lithium compound oxideshowed that it contained 7.3% Li, 31.1% Ni, 16.0% Mn, 8.4% Co, and 3.8%W and had a molar ratio of Li/(Ni+Mn+Co+W) of 1.07.

An X-ray diffraction pattern of the lithium compound oxide using Cu Kαrays is shown in FIG. 3. It can be seen that the X-ray diffractionpattern includes, in addition to main diffraction peaks capable of beingindexed under space group R3-m, secondary diffraction peaks at 21.04°and 23.44° assigned to Li₂WO₄ of the rhombohedral system.

The results of measurements with a test battery prepared in the samemanner as described in Example 1 and the results of DSC measurements areshown in Table 1 and FIG. 7.

Comparative Example 1

A mixture obtained by adding 9.56 kg of lithium hydroxide (anhydrous) to30.44 kg of a Ni—Mn—Co compound oxide (Ni:Mn:Co=0.56:0.30:0.14) followedby mixing uniformly was calcined at a temperature of 900° C. in the samemanner as described in Example 1 to give 36.92 kg of a lithium compoundoxide.

Analysis of the composition of the resulting lithium compound oxideshowed that it contained 7.4% Li, 32.8% Ni, 16.2% Mn, and 8.5% Co inmass % and had a molar ratio of Li/(Ni+Mn+Co) of 1.07.

An X-ray diffraction pattern of the lithium compound oxide using Cu Kαrays is shown in FIG. 4. It can be seen that the X-ray diffractionpattern includes only main diffraction peaks capable of being indexedunder space group R3-m.

The results of measurements with a test battery prepared in the samemanner as described in Example 1 and the results of DSC measurements areshown in Table 1 and FIG. 7.

Comparative Example 2

A mixture obtained by uniformly mixing 32.65 kg of commerciallyavailable nickel hydroxide and 7.35 kg of lithium hydroxide (anhydrous)was calcined at a temperature of 700° C. in the same manner as describedin Example 1 to give 31.92 kg of lithium nickelate.

Analysis of the composition of the resulting lithium nickelate showedthat it contained 6.8% Li, 59.4% Ni, and 0.7% Co in mass % and had amolar ratio of Li/(Ni+Co) of 0.96.

An X-ray diffraction pattern of the product using Cu Kα rays is shown inFIG. 5. It can be seen that the X-ray diffraction pattern includes onlymain diffraction peaks capable of being indexed under space group R3-m.

The results of measurements with a test battery prepared in the samemanner as described in Example 1 and the results of DSC measurements areshown in Table 1 and FIG. 7.

Comparative Example 3

A mixture obtained by uniformly mixing 31.66 kg of commerciallyavailable cobalt tetraoxide and 8.34 kg of lithium hydroxide (anhydrous)was calcined at a temperature of 900° C. in the same manner as describedin Example 1 to give 36.08 kg of lithium cobaltate.

Analysis of the composition of the resulting lithium cobaltate showedthat it contained 7.0% Li and 59.7% Co in mass % and had a molar ratioof Li/Co of 1.00.

An X-ray diffraction pattern of the product using Cu Kα rays is shown inFIG. 6. It can be seen that the X-ray diffraction pattern includes onlymain diffraction peaks capable of being indexed under space group R3-m.

The results of measurements with a test battery prepared in the samemanner as described in Example 1 and the results of DSC measurements areshown in Table 1 and FIG. 7.

Comparative Example 4

In exactly the same manner as described in Example 1 except that 9.56 kgof lithium hydroxide (anhydrous) and 0.88 kg of aluminum hydroxide wereadded to 29.57 kg of a Ni—Mn—Co compound oxide(Ni:Mn:Co=0.56:0.30:0.14), 37.12 kg of a lithium compound oxide wereobtained.

The lithium compound oxide had a compositional formula and a compositionas shown in Table 1. The results of measurements with a test batteryprepared in the same manner as described in Example 1 and the results ofDSC measurements are shown in Table 1 and FIG. 7.

Comparative Example 5

In exactly the same manner as described in Example 1 except that 9.54 kgof lithium hydroxide (anhydrous) and 0.93 kg of titanium oxide wereadded to 29.52 kg of a Ni—Mn—Co compound oxide(Ni:Mn:Co=0.56:0.30:0.14), 36.58 kg of a lithium compound oxide wereobtained.

The lithium compound oxide had a compositional formula and a compositionas shown in Table 1. The results of measurements with a test batteryprepared in the same manner as described in Example 1 and the results ofDSC measurements are shown in Table 1 and FIG. 7.

Comparative Example 6

To 29.84 kg of a commercially available nickel hydroxide, 8.01 kg oflithium hydroxide (anhydrous) and 2.15 kg of tungsten trioxide wereadded and uniformly mixed. The lithium hydroxide was the same finepowder of 20 μm or smaller as used in Example 1, and the tungstentrioxide was also a fine powder of 1 to 20 μm. The resulting powdermixture was placed in an aluminum vessel having a purity of 99.8% andcalcined for more than several hours at a temperature of 950-750° C. ina pure oxygen atmosphere to give 32.82 kg of a lithium compound oxide.

The lithium compound oxide had a compositional formula and a compositionas shown in Table 1. The results of measurements with a test batteryprepared in the same manner as described in Example 1 and the results ofDSC measurements are shown in Table 1 and FIG. 7.

TABLE 1 Battery Test DSC Results Metal Contents (mass %) Initial CycleExotherm Calorific Other Capacity Life Peak Value No. CompositionalFormula Li Ni Mn Co Metal (mAh/g) (%) Temp. (J/g) Example 1Li_(1.04)Ni_(0.53)Mn_(0.30)Co_(0.14)W_(0.03)O₂ 7.1 30.8 16.0 8.3 W: 4.6157 95 312° C. 295 2 Li_(1.05)Ni_(0.53)Mn_(0.30)Co_(0.14)Mo_(0.03)O₂ 7.230.9 16.1 8.3 Mo: 2.8 156 94 296° C. 366 3Li_(1.07)Ni_(0.54)Mn_(0.30)Co_(0.14)W_(0.02)O₂ 7.3 31.1 16.0 8.4 W: 3.8161 94 311° C. 363 Comparative Example 1Li_(1.07)Ni_(0.56)Mn_(0.30)Co_(0.14)O₂ 7.4 32.8 16.2 8.5 — 165 89 314°C. 619 2 Li_(0.96)Ni_(0.99)Co_(0.01)O₂ 6.8 59.4 0.0 0.7 — 180 80 220° C.1236 3 Li_(1.00)Co_(1.00)O₂ 7.0 0.0 0.0 59.7 — 150 95 250° C. 517 4Li_(1.05)Ni_(0.54)Mn_(0.30)Co_(0.14)Al_(0.03)O₂ 7.4 32.0 16.6 8.4 Al:0.8 154 89 312° C. 643 5 Li_(1.05)Ni_(0.54)Mn_(0.30)Co_(0.14)Ti_(0.03)O₂7.4 31.8 16.6 8.4 Ti: 1.4 157 92 302° C. 656 6Li_(1.08)Ni_(0.97)W_(0.03)O₂ 7.0 55.0 — — W: 5.1 137 89 216° C. 704

As can be seen from the results of the battery test shown in Table 1,the initial capacity of lithium nickelate of Comparative Example 2 wasconsiderably higher than that of lithium cobaltate of ComparativeExample 3, which is used as a positive active material in currentpractical lithium ion secondary batteries. However, lithium nickelatehad a low DSC exotherm peak temperature of 220° C. with a very largecalorific value of 1236 J/g in a charged state. Thus, the thermalstability of lithium nickelate in a charged state was significantlylower than that of lithium cobaltate.

In contrast, the lithium compound oxide of Comparative Example 1 inwhich Mn and Co were added to lithium nickelate maintained an initialcapacity higher than that of lithium cobaltate and had a DSC exothermpeak temperature shifted to a higher temperature than that of lithiumnickelate with an approximately halved calorific value. Thus, thelithium compound oxide of Comparative Example 1 was improved to someextent with respect to thermal stability in a charged state. However,its calorific value was still larger than that of lithium cobaltate ofComparative Example 3, so the improvement in thermal stability wasinsufficient.

The lithium compound oxides of Examples 1 to 3, in which a very smallamount of W or Mo at a molar ratio of 0.02 to 0.03 in theircompositional formula was added to the Mn, Cu-containing lithiumnickelate of Comparative Example 1 according to the present invention,had DSC calorific values in a charged state which were reduced toapproximately half (48-59%) the value in Comparative Example 1.Nevertheless, these oxides had initial capacities which were stillmaintained at substantially the same level as the material ofComparative Example 1, i.e., which were higher than that of lithiumcobaltate of Comparative Example 3. The DSC exotherm peak temperaturesof these oxides were nearly the same as that of Comparative Example 1.Therefore, the addition of a very small amount of W and/or Mo accordingto the present invention has the surprising thermal stabilization effectthat it is possible to approximately halve the calorific value withoutsignificantly varying the initial capacity or shifting the exotherm peaktemperature.

In accordance with the present invention, it is made possible to providehigh-performance positive active materials which are improved in bothcapacity and thermal stability in a charged state with lower costs dueto a lower Co content, compared to lithium cobaltate (ComparativeExample 3) which is the current practical positive active material.Specifically, they have a higher initial capacity, a DSC exotherm peaktemperature increased by more than about 50° C., and a calorific valuesuppressed to about 57-71% compared to lithium cobaltate. Compared tolithium nickelate (Comparative Example 2), the calorific values of thepositive active materials according to the present invention in acharged state are significantly reduced to 24-30% the value of lithiumnickelate.

The exothermic heat appears to be concerned with a reaction between theelectrolytic solution and oxygen, which is generated from the positiveactive material in a charged state by decomposition occurring at a hightemperature. With a positive active material according to the presentinvention, it is presumed that the decomposition temperature shifts to ahigher temperature and that the amount of oxygen generated by thedecomposition is significantly small.

With respect to cycle life, the positive active materials of Examples 1to 3 according to the present invention had an excellent cycle lifecomparable to that of lithium cobaltate of Comparative Example 3 whichis now in practical use. Since the cycle life of these active materialswas improved over the material of Comparative Example 1 which did notcontain W and Mo, it can be concluded that the addition of W or Mo iseffective for improving not only thermal stability but also cycle life.The positive active material of lithium nickelate of Comparative Example2 had a poor cycle life.

As shown in FIGS. 1 to 3, the lithium compound oxides obtained inExamples 1 to 3 gave X-ray diffraction patterns including one or morepeaks assigned to the rhombohedral system of a compound oxide of Li withthe added W or Mo (Li₂WO₄, Li₄MoO₅). Thus, it can be concluded that atleast part of the added W or Mo is present as a compound oxide with Liand forms a different phase from the hexagonal phase. Although themechanism is not clear, the phase of the compound oxide of Li with W orMo seems to contribute to the improved thermal stability of a positiveactive material according to the present invention.

With the lithium compound oxides of Comparative Examples 4 and 5 inwhich Al or Ti was added, in place of W or Mo, at the same molar ratioof 0.03 as the Examples, the DSC calorific values in a charged statewere approximately at the same level as or slightly inferior to (largerthan) the value in Comparative Example 1 containing no additive element.Thus, the effect on improving thermal stability in a charged stateachieved by addition of a small amount of an element to the compositionof Comparative Example 1 in accordance with the present invention isinherently applicable when the added element is W or Mo and it cannot beobtained when another element is added.

The material of Comparative Example 6 in which a small amount of W wasadded to lithium nickelate was not satisfactory in both batteryproperties and thermal stability. Thus, it is entirely impossible toachieve the object of the present invention by addition of W alone topure lithium nickelate which is not in a compound oxide.

Industrial Applicability

The present invention provides a high-performance positive activematerial which has a higher initial capacity and higher thermalstability in a charged state compared to LiCoO₂ (lithium cobaltate) usedas a positive active material in current practical lithium ion secondarybatteries. The positive active material according to the presentinvention can be prepared less expensively than the current positiveactive material. Therefore, the positive active material is useful invarious nonaqueous electrolyte secondary batteries including lithium ionsecondary batteries, thereby contributing to improvement in performanceand reduction in costs of nonaqueous electrolyte secondary batteries andto development of electric cars equipped with large-sized nonaqueouselectrolyte secondary batteries.

It should be understood by those skilled in the art that the presentinvention is not restricted to the specific embodiments described aboveand various modifications of the embodiments described above can be madewithout departing from the scope of the present invention.

What is claimed is:
 1. A positive active material for nonaqueous electrolyte secondary batteries, characterized in that it is comprised of a lithium compound oxide of the formula: Li_(a)Ni_(b)Co_(c)Mn_(d)M_(e)O₂  (1) where M stands for one or two of W and Mo, 0.90≦a≦1.15, 0<b<0.99, 0<c≦0.5, 0<d≦0.5, 0 <c+d≦0.9, 0.01≦e≦0.1, and b+c+d+e=1, and in that the lithium compound oxide gives an X-ray diffraction pattern including a diffraction peak or peaks assigned to a compound oxide of Li and W and/or a compound oxide of Li and Mo, in addition to main diffraction peaks assigned to a hexagonal crystal structure.
 2. The positive active material for nonaqueous electrolyte secondary batteries according to claim 1, wherein “a” is 0.95≦a≦1.10.
 3. The positive active material for nonaqueous electrolyte secondary batteries according to claim 1, wherein “c” is 0.05≦c≦0.4.
 4. The positive active material for nonaqueous electrolyte secondary batteries according to claim 1, wherein “d” is 0.01≦d≦0.3.
 5. The positive active material for nonaqueous electrolyte secondary batteries according to claim 1, wherein “c” and “d” are 0<c+d≦0.7.
 6. The positive active material for nonaqueous electrolyte secondary batteries according to claim 1, wherein “e” is 0.01≦e<0.5.
 7. The positive active material for nonaqueous electrolyte secondary batteries according to claim 1, wherein the diffraction peak or peaks assigned to a compound oxide of Li and W and/or a compound oxide of Li and Mo is a peak or peaks assigned to rhombohedral Li₂WO₄ and/or Li₄MoO₅.
 8. A positive electrode mixture for nonaqueous electrolyte secondary batteries predominantly comprising a positive active material according to claim
 1. 9. The positive electrode mixture for nonaqueous electrolyte secondary batteries according to claim 8 which further contains a binder and a conducting additive.
 10. A nonaqueous electrolyte secondary battery comprising a negative electrode comprised of lithium metal or a substance capable of reversibly absorbing and desorbing Li or Li ions and a positive electrode comprised of a mixture according to claim
 8. 